Do miles awarded expire once they are in a member's account?

No, AAdvantage miles do not expire as long as you have qualifying account activity at least once every 18 months. Qualifying account activity is defined as any AAdvantage mileage accrual or AAdvantage award redemption. 

For more member program information, please refer to AA.com, http://www.aa.com/i18n/AAdvantage/programInformation/main.jsp?from=Nav